We have developed a method that allows transfer of testicular cells from a fertile male to the seminiferous tubules of an infertile male. Following transfer, the donor cell population establishes normal spermatogenesis in the recipient testes, and mature spermatozoa are produced and found in the epididymis. In the most successful transplantations, the recipient male becomes fertile, and the donor cell haplotype if found in progeny of the male. We have used this approach to demonstrate that spermatogonial stem cells can be cryopreserved, thus making individual male germ lines immortal. In addition, transplanted rat testis cells will generate spermatogenesis in the seminiferous tubules of immunodeficient mice and produce rat spermatozoa, suggesting that xenogeneic spermatogenesis may be possible for other species. This testis call transplantation technique provides an opportunity to perform experiments with male germ cells that previously were difficult or impossible. In this project, we have three specific aims. 1. To increase the efficiency with which donor cells colonize and repopulate the seminiferous tubules of recipient mice. 2. To grow in vitro and expand the number of the testicular stem cells responsible for repopulating the recipient testis. 3. To modify the genetic characteristics of spermatogonial stem cells and recover the new haplotype in progeny. The studies proposed will make spermatogonial transplantation useful for a wide range of applications in biology, medicine and agriculture. In addition, the knowledge gained will help alleviate certain forms of male infertility and help in the development of new contraceptive approaches.